GB2109636A

GB2109636A - Electromagnetic switching device for a starting motor

Info

Publication number: GB2109636A
Application number: GB08231057A
Authority: GB
Inventors: Roberto Fusetti
Original assignee: Fabbrica Italiana Magneti Marelli SpA; Magneti Marelli SpA
Current assignee: Industrie Magneti Marelli SRL; Marelli Europe SpA
Priority date: 1981-10-29
Filing date: 1982-10-29
Publication date: 1983-06-02
Also published as: ES516917A0; FR2515744A1; IT1140007B; IT8124754A0; ES8308140A1; DE3239672A1; FR2515744B3

Abstract

An internal combustion engine is started by a starting motor 1 under the control of an electromagnetic device which is energisable upon closure of an ignition switch I to both engage a coupling between the engine and the starting motor and to actuate a contact assembly including contacts 35, 31, 36 closing a circuit path for energising the motor. The electromagnetic device has two windings 12, 13 and the contact assembly includes contacts 27, 21, 28 and 32, 31, 33 connecting the windings in parallel for initial energisation (operating step) and changing over the windings to series- connection while the starting motor is energised (holding step). Return of the electromagnet, coupling and contact assembly to the rest condition (release step) is done by spring action upon opening the ignition switch to deenergise the windings. The contact assembly is in two parts with respective actuators actuated by the electromagnet core armature 8 with lost motion to ensure a correct operating sequence. <IMAGE>

Description

SPECIFICATION Electromagnetic device for a starting motor This invention relates to an electomagnetic device for use with the starting motor of internal combustion engine or other heat engine, particularly in motor vehicles. The invention also relates to a power unit whose prime mover is an internal combustion engine or other heat engine and which has an electrically energisable starting motor for the engine and an electromagnetic device energisable to initiate starting of the engine. In such a power unit the electromagnetic device has a movable core member which is mechanically linked to means for selectively coupling the starting motor to the engine such that displacement of the core member under the action of an induced magnetic field effects the coupling. Contacts actuable by the core also energise the starting motor.The invention will be discussed with particular reference to engines of motor vehicles. In starting systems for such engines it is known to provide the electromagnetic device, or electromagnet as it may be more simply called, with a pair of energisable windings to provide and control the magnetic field to act upon the core.

In known electromagnets of this kind, there are three distinct operating steps, that is, an initial or take-off step to actuate the core from its inoperative position, intermediate or holding step to maintain the core actuated (operative position), and a final or release step to restore the core to its inoperative position once the engine starts. The two windings are normally referred to as take-off winding and holding winding, for reasons that will appear in the following discussion of the construction and operation of the known dual winding electromagnets.

The take-off step occurs as soon as the ignition lock key is turned to close the ignition switch.

Such operation energises the two windings which, at this initial step, are parallel connected, developing a magnetic field that causes the displacement of the movable core of the electromagnet and parts connected to it from a rest or inoperative position to a working position. On the one hand, such parts include a clutch device providing a mechanical coupling between the starting motor and the engine and, on the other hand, the closure of an electric circuit path connecting the supply battery to the starting motor. It should be noted that, at the take-off step, the resulting magnetic field is essentially generated by one winding-the take-off winding-since the other-the holding winding-carries only a relatively low current.

The holding step coincides with the time for which the starting motor is supplied with current to start the engine. During this step, the core and associated parts are maintained in working position due to energisation of only the holding winding, the other winding (take-off winding) being cut-out, as by shorting.

Finally, the release step occurs as soon as the ignition lock key returns to the rest or inoperative position. The magnetic field acting on the core is cancelled and the core and connected parts return to the rest or inoperative position. The magnetic field cancellation occurs in that the two windings, series-connected at this step, have the same number of turns but carry currents in reverse direction as regards field generation, and accordingly the total ampere-turns are zero.

The dual winding electromagnets of the above described type suffer from some disadvantages and limitations. Firstly, the two windings are insufficiently used both at the take-off step and the holding step. As above mentioned the holding winding at the take-off step carries low current, and accordingly the magnetic field required for causing the displacement of the core is nearly entirely generated by the ampere-turns of the take-off winding. However, since this winding needs to have the same number of turns as the holding winding for cancellation of the respective ampere-turns at the release step, it necessarily uses wire of large cross-section to take the high current required for the take-off step, with a resulting increase in weight, overall size and electromagnet cost.

The low current through the holding winding at the take-off step is the same as that passing through this winding during the holding step and the value of which must be limited. This is because a current of high value would damage connected electric circuits, and in addition cause undue discharge of the battery, bearing in mind that the holding step may last for a relatively long time, in the order of some tens of seconds.

The insufficient use of the two windings is apparent at the holding step because only the holding winding is energised, the other winding being shorted and accordingly not used.

Finally, in the prior art electromagnets, the condition that at release step the magnetic fields due to the two windings cancel one another, to allow the core and connected parts to return to the rest or inoperative position, requires that the number of turns of the take-off winding is the same as that of the holding winding. Such a condition is evidently a limiting restraint on design of the electromagnet.

There will be described hereinafter an embodiment of the present invention applied to the starting system for an internal combustion engine in which a dual winding electromagnet is constructed and operated in such way that the two windings are exploited to greater extent, enabling the electromagnet to be made lighter and of less overall size than prior electromagnets developing the same power. The weight can be further reduced by using aluminium rather than copper windings. The preferred embodiment is also capable of maintaining the core and associated driven or controlled parts in the working position if some lowering of the supply voltage should occur. Finally, the number of turns in the windings do not necessarily have to be equal.

Broadly stated, the present invention provides in one aspect an electromagnetic device having a pair of windings energisable to provide a magnetic field to act upon a movable core member, and comprising a contact assembly actuable by displacement of the core member and having contacts electrically connected to said windings and having contacts for controlling a further circuit path, said contact assembly being biased to a first position in which said windings are connected in parallel ready to receive energising current and said further circuit path is open, and being actuable in response to a displacement of the core member upon energisation of the windings to a second position in which said windings are connected in series to provide a holding magnetic field to maintain the core in its displaced condition and said further circuit path is closed, said contact assembly being restored to the first position upon de-energisation of said windings.

In another aspect the invention provides a power unit whose prime mover is an internal combustion engine or other heat engine and which has an electrically energisable starting motor for the engine and and electromagnetic device energisable to initiate starting of the engine, said electromagnetic device having a movable core member and a pair of windings energisable by closure of an ignition switch to provide a magnetic field to act upon the core member, the core member being mechanically linked to means for selectively coupling the starting motor to the engine such that displacement of the core member under the action of such magnetic field effects said coupling, wherein the electromagnetic device further comprises:: a contact assembly acutable by displacement of the core member and having contacts electrically connected to said windings and providing a further circuit path for supplying energising current to the starting motor, said contact assembly being biased to a first position in which said windings are connected in parallel ready to receive energising current and said further circuit path is open, and being actuable, in response to a displacement of the core member upon energisation of the windings, to a second position in which said windings are connected in series to provide a holding magnetic field to maintain the core member in its displaced condition and in which said further circuit path is closed to energise the starting motor; and said contact assembly being restored to the first position upon de-energisation of the windings.

By use of the invention in the ignition system of an engine both windings are used in take-off step, where such windings are parallel connected for initial energisation, and both are used at the holding step, where these windings are series connected. Both are de-energised at release step.

Because the two windings are both used in these two steps, it becomes possible to use copper wires of reduced cross-section and/or of lesser numbers of turns than previously, with the resulting advantage of reduction in weight and overall size of the electromagnet.

Alternatively, for the same overall size, the windings may be of aluminium.

For a given current, the two series-connected windings will generate double the magnetic field of a single winding in the holding step, because of twice the number of ampere-turns, which better ensures that the core and other actuated parts are in the working position even if a lowering of the supply voltage occurs.

At the release step, the magnetic field disappears because the two windings are not energised. Thus, the return to rest or inoperative position is not dependent cn the controlled energisation of the windings and no specific relation is required between the number of turns for the two windings, as was previously the case.

The number of turns may be accordingly chosen freely in connection with other design requirements.

A preferred embodiment of the invention applied to the ignition and starting motor system of an internal combustion engine will now be described with reference to the accompanying drawings, in which: Figure 1 is a partly cut away view showing the starting motor, the clutch for coupling it to the engine, an electromagnet for actuating the clutch and energising the starting motor; Figure 2 is an enlarged longitudinal, sectional view showing the electromagnet of Figure 1; Figure 3 is a fragmentary sectional view, taken along the line Ill-Ill, of the contact assembly of the electromagnet shown in Figure 2; and Figure 4 shows a sequence a) to d) of circuit diagrams for the electromagnet at different operating steps.

The assembly shown in Figure 1 comprises a D.C. electric motor 1, hereinafter referred to as the starting motor, and a dual winding electro magnet 2 and means, i.e. a clutch mechanism for selectively coupling the shaft of motor 1 to the internal combustion engine (not shown) which is the prime mover of a power unit of a motor vehicle for example.

At a splined portion, the motor shaft 3 has a slidable sleeve 4 mounted thereon, and at the drive or control end a pinion 5 intended to mesh with a crown or ring gear 6 of the internal combustion engine.

The sleeve and pinion are coupled to each other by means of a free wheel device known per se. The electromagnet 2 comprises an axially movable core 8 connected to the sleeve 4 by means of a pivotally-mounted lever 9. The core 8 provides a magnetic circuit with a fixed core 10 and a ferromagnetic body or shell 1 Within the shell two coaxial windings are arranged about the two cores, the inner being denoted 1 2 and the outer 13.

One electrical (input) terminal 14 of the electromagnet is directly connected to the vehicle battery (not shown in Fig. 1). A second (output) terminal 15 is connected to motor 1, the terminals being secured to an insulating closure cap 1 6 housing a contact assembly to be described.

In operation, which will be described in greater detail below, the energisation of the electromagnet first causes an axial displacement of the core 8 and therewith the sleeve-pinion assembly 4, 5, so that the pinion meshes with crown or ring gear 6; and then the closure of the supply circuit for motor 1 which drives the engine through the pinion-ring gear coupling. When the engine is started, the electromagnet is de-energised and the various members are restored to the rest or inoperative position shown in Figure 1.

The current supply to windings 12, 13 and motor 1 is provided with switching means under the control of core 8. The switching means comprises a contact assembly actuated by an actuator arrangement 1 7 which is linearly movable, though rotatable arrangements could be used. The whole actuator and contact assembly is more clearly shown in Figures 2 and 3 and its electrical arrangement and operation in Figure 4, in which the corresponding parts are denoted by the same references. The arrangement 1 7 comprises an elongate body having an insulating rod 1 8 terminating adjacent the movable core with a head 18', and an insulating sleeve 19 freely surrounding said rod and sliding in a hoie 20 of the fixed core 1 0.Both the rod and sleeve are lengthwise or axially movable, and in the inoperative position the head 18' is spaced apart by a length or distance H from the adjacent end of the sleeve 1 9 through which the rod extends.

When the electromagnet windings are energised the core 8 is drawn into the windings by the resultant magnetic field and eventually strikes head 1 8 to displace it to the right (as seen in Fig.

1). There some lost motion before the head 1 8 is displaced due to the spacing between it and the core 8 and a further lost motion in actuating the sleeve 1 9 due to the spacing H about which more is said below.

The contact assembly comprises two movable contacts 21 and 31 carried by rod 18 and sleeve 1 9 respectively and each of these is located between two pairs of fixed contacts so as to bridge one pair in the inoperative position and the other pair in the operative or working position.

The fixed contacts are discussed further below.

The first movable contact comprises an electrically conductive element in the form of an apertured metal plate 21, through the aperture of which the rod 1 8 extends. The plate 21 is resiliently clamped between two cups 22, 23, slidably mounted on rod 1 8 and spring-loaded to be located by an abutment of the rod, provided for example by a Seiger ring (circlip) 24.

The resilient clamping of plate 21 is obtained by means of two opposing springs 25, 26, mounted about the rod, of which one spring 25 acts between the bottom of a fixed part of the electromagnet, namely cap 16, and facing cup 22, while the other spring 26 acts between cup 23 and nearer end of sleeve 1 9. These springs are preloaded, with spring 25 having a larger load than spring 26.

The above mentioned plate 21 is located between and co-operates with two pairs of fixed contacts, that is a lower pair 27, 28 and an upper pair 29, 30, as seen in Figure 3. The contacts of each pair are symmetrically located with respect to the axis of the contact assembly, i.e. the axis of rod 1 8 and sleeve 1 9. Under rest or inoperative conditions, spring 25 maintains the plate 21 in contact with an electrically bridging the lower pair of contacts 27, 28, while in the working condition, that is, when said rod 1 8 is driven upwardly in Figures 2 and 3 by the movable core 8, the plate 21 is in contact with and electrically bridging the upper pair of contacts 29, 30.

The second movable contact also comprises an electrically conductive element in the form of an apertured metal plate 31. The sleeve 19 (and the rod therein) extends through the plate aperture.

The plate 31 is located between and co-operates with two pairs of fixed contacts, that is a lower contact pair 32, 33 (as seen in Fig. 2) secured to the core 10 through the insulating terminal board 34, and an upper contact pair 35, 36 respectively connected to the terminals 14 and 1 5.

Under the rest or inoperative condition, the plate 31 is maintained in contact with and electrically bridging the lower contact pair 32, 33 through the action of spring 26 acting thereon via the end portion of sleeve 1 9 further from the head 18'. This end portion has an annular interior recess providing a seat for spring 26. Under the working condition, plate 31 is in contact with and electrically bridging the upper contact pair 35, 36 and closes a circuit path connecting battery B to motor 1 as will be seen from Fig. 4.

The contact plate 31 is located between two abutments of sleeve 19, one being an outwardly directed flange 1 9' at the end of the sleeve and the other being provided by a Sieger ring (circlip) 38. The plate 31 is held against the flange 1 9' by a third spring 37 arranged about sleeve 19. In the inoperative condition the plate 31 is biased by the action of springs 25 and 26 onto the lower fixed contacts 32, 33, the spring 37 serving to urge the plate 31 against the upper fixed contacts 35,36 in the working condition.

In order that the operation be understood, it should now be noted that the displacement h that plate 21 travels to come in contact with the upper contacts 29, 30 is less than the displacement H that the head 18' of rod 18 travels to come in contact with the opposite end of sleeve 1 9 and move the plate 31. There is thus a lost motion between the rod and the sleeve which ensures that the contact change-over effected by plate 21 is done before that effected by plate 31.

The electrical connection for the four pairs of fixed contacts 27, 28 and 29 in one change-over switch and 32, 33 and 35, 36 in the other change-over switch is shown in the diagrams of Figure 4. This figure shows the ignition circuit including the battery B, the ignition switch I and the electromagnet windings interconnected with the contact assembly. The sequence of diagrams of Figure 4 shows the electromagnet under rest or inoperative condition (Fig. 4a), at take-off step (Fig. 4b), at holding step (Fig. 4c) and at release step (Fig. 4d).

It will be seen in Fig. 4a that one end of winding 12 is connectable to battery B through the switch I operated by the ignition lock key and the other winding end is connected to ground by plate 31 bridging the lower pair of contacts 32, 33 associated with the sleeve. The side of switch I connected to winding 1 2 is also connected to one end of winding 13 by the plate 21 bridging the lower contact pair 27, 28 associated with the rod 18. The other end of winding is connected to ground. The one end of winding 13 is also connected to contact 30 of the upper pair of fixed contacts associated with the rod 18, the other contact 29 of this pair being connected to contact 32 and the other end of winding 12.Finally the upper pair of fixed contacts 35, 36 associated with the sleeve are connected directly to terminals 14 and 1 5 as already indicated to control the supply of current from the battery B to the motor 1.

The operation of the electromagnet will now be described with particular reference to the diagrams of Figure 4.

Rest (inoperative) conditions (Fig. 4a) The circuit is as described above with the rod 18 and sleeve 1 9 spring-urged into the rest condition spaced from movable core 8 which itself is in the rest condition since the windings of the electromagnet are not energised. It will be seen that by bridging of the respective lower pairs of fixed contacts, 27, 28 and 32, 33, the windings 12 and 13 are connected in parallel ready to receive current. The circuit path for energising the motor 1 through contacts 35, 36 is open.

Take-off step (Fig. 4b) This step initiates with closing of switch I and lasts for a very short time.

The two parallel-connected windings 12 and 13 are energised from battery B through switch I.

The arrows by the windings indicate the directions for the current, for which the magnetic fields from the windings add.

The energisation of the two windings generates a magnetic field which causes the displacement of movable core 8 towards fixed core 1 0.

As this displacement takes place, the core 8 first engages head 18' of rod 18', then continuing its stroke or travel it provides through displacement of rod 1 8 the changing-over of the plate 21 from bridging lower contacts 27, 28 to bridging the upper or working contacts 29, 30.

The displacement of plate 21 is done under the thrust of spring 26.

At the same time, through lever 9 and sliding sleeve 4, the core 8 first controls the partial meshing of pinion 5 with ring gear 6, and then during the stroke or travel H and subsequent displacement the full meshing of the gears.

The subsequent displacement now displaces the plate 31 onto supply contacts 35, 36 and motor 1 is connected to battery B and accordingly starts driving the engine.

It should be pointed out that both windings 12 and 1 3 contribute in the initial step to generate the magnetic field, so that both windings can be exploited to the same extent. This has the advantage of a possible reduction in weight and overall size of the electromagnet as compared to using one winding, or virtually one winding, only at this stage. Aluminium windings can be used, thus providing a further reduction in weight.

It should also be pointed that, since the stroke or travel h of head 18' to change-over bridging contact 21 is less than stroke H to initiate change-over of bridging contact 31, the continuity of the magnetic field is assured during the complete change-over of the windings, since winding 12 continues to be energized while plate 21 is changing over. Thus, contacts 32, 33 cannot be opened prior to closing of contacts 29, 30.

Holding step (Fig. 4c) At this step, the two windings 1 2 and 13 are now series-connected through plate 21 bridging contacts 29, 30 and motor 1 is energised through plate 31 bridging contacts 35,36.

The currents in the two windings (see arrows) are in the same direction, so that the total ampere-turns are the sum of the ampere-turns in the two windings. This provides better holding even if the battery voltage should drop.

This assures that all of the parts controlled or driven by core 8 are maintained in the operating position.

Release step (Fig. 4d) This step is carried out when the switch I opens upon release of the ignition key.

At this time, the two windings, still seriesconnected, are no longer energised and as a result the holding magnetic field disappears and rod 1 8 and sleeve 1 9 are brought back to rest or inoperative position (Fig. 4a) under the action of spring 25, 26 (Figs. 3 and 4), opening the energising circuit for motor 1. At the same time the pinion 5 is disconnected from ring gear 6, but by now the engine has started.

As above disclosed, the disappearance of the magnetic field occurs by lack of current in the two windings rather than by cancellation of opposing fields. Therefore, unlike prior electromagnets, no relationship has to be fulfilled between the number of turns in the two windings to obtain at release step the disappearance of the two magnetic field. Moreover, in the absence of any restraints, the wires in the two windings may be of the same diameter.

The advantages provided by an electromagnet as described are first concerned with a reduction in weight, which goal is nowadays pursued by automobile manufacturers to obtain a resulting reduction in fuel consumption. However, advantages are also provided in reduction of overall size and costs.

With electromagnets of the structure now disclosed using copper wires, a reduction in weight and overall size of 16% and 17% respectively have been obtained under the same power. On the other hand, by using aluminium wires, the reduction in weight is of 19%, with unaltered overall size.

The above data relates to the unit or assembly shown in Figure 2.

The economic advantages are mainly related to the lower weight and hence lower cost for the wires; but also the manufacturing process for the coils or bobbins is simpler. Thus, in the absence of any ratio between the number of turns for the two windings, no limitations exist in tolerances on the number of turns for such windings, and a wire of the same diameter can be also used for both windings.

The above advantages are provided without jeopardising the ratio between the currents drawn at holding and take-off steps, which remains at a value of 1:4, as averaged in conventional electromagnets.

Finally, in the foregoing description, reference was made to electric connections comprising conducting plates controlled by a moving element comprising a rod and a sleeve. However, it is clear that there are other possibilities for the number and arrangement of the fixed contacts and structure of the moving element.

Claims

1. An electromagnetic device having a pair of windings energisable to provide a magnetic field to act upon a movable core member, and comprising a contact assembly actuable by displacement of the core member and having contacts electrically connected to said windings and having contacts for controlling a further circuit path, said contact assembly being biased to a first position in which said windings are connected in parallel ready to receive energising current and said further circuit path is open, and being actuable in response to a displacement of the core member upon energisation of the windings to a second position in which said windings are connected in series to provide a holding magnetic field to maintain the core in its displace condition and said further circuit path is closed, said contact assembly being restored to the first position upon de-energisation of said windings.

2. A power unit whose primer mover is an internal combustion engine or other heat engine and which has an electrically energisable starting motor for the engine and an electromagnetic device energisable to initiate starting of the engine said electromagnetic device having a movable core member and a pair of windings energisable by closure of an ignition switch to provide a magnetic field to act upon the core member, the core member being mechanically linked to means for selectively coupling the starting motor to the engine such that displacement of the core member under the action of such magnetic field effects said coupling, wherein the electromagnetic device further comprises:: a contact assembly actuable by displacement of the core member and having contacts electrically connected to said windings and providing a further circuit path for supplying energising current to the starting motor, said contact assembly being biased to a first position in which said windings are connected in parallel ready to receive energising current and said further circuit path is open, and being actuable in response to a displacement of the core member upon energisation of the windings, to a second position in which said windings are connected in series to provide a holding magnetic field to maintain the core member in its displaced condition and in which said further circuit path is closed to energise the starting motor; and said contact assembly being restored to the first position upon de-energisation of the windings.

3. An electromagnetic device as claimed in Claim 1 or a power unit as claimed in Claim 2 in which said contact assembly comprises two contact actuating members, a first of which is actuable to change over said windings from a parallel to a series connection and the second of which is actuable to close said further circuit path subsequent to the change over effected by actuation of the first member.

4. An electromagnetic device or a power unit as claimed in Claim 3 in which said second member is actuable with lost motion relative to said first member.

5. An electromagnetic device or a power unit as claimed in Claim 4 in which said first member has an elongate body and a head portion at one end of the body upon which said core acts to displace the body lengthwise and said second member comprises a sleeve through which the body of the first member extends, the end of which adjacent said head portion is spaced from the head portion and which is displaceable lengthwise upon being contacted by the head portion.

6. An electromagnetic device or a power unit as claimed in Claim 5 comprising resilient means acting between said first and second members to bias the second member toward the head portion of the first member.

7. An electromagnetic device or a power unit as claimed in Claim 6 in which said contact assembly is biased to said first position by resilient means acting between a fixed part and said first actuating member.

8. An electromagnetic device or a power unit as claimed in Claim 4, 5, 6 or 7 comprising first and second conductive plates mounted on said first and second actuating members respectively, each plate having an aperature therein through which the respective actuating member extends, the plates serving to provide bridging contacts between pairs of fixed contacts, the contacts of each pair being symmetrically located with respect to the axis of the aperature.

9. An electromagnetic device or a power unit as claimed in Claim 8 in which each plate is located between two pairs of such fixed contacts, bridging one pair in said first position and the other pair in said second position, and the displacement of the first conductive plate from said first to said second position is less than the lost motion between the first and second actuating members.

10. An electromagnetic device or a power unit as claimed in Claim 7, comprising first and second conductive elements mounted on said first and second actuating members respectively, each element having an aperture through which the respective actuating member extends, the elements serving to provide bridging contacts between pairs of fixed contacts, the contacts of each pair being symmetrically located with respect to the axis of the aperture, and further comprising a pair of apertured members located axially on each side of said first element and separated by a fixed abutment carried by the body portion of the first actuating member, said resilient means acting between the first and second actuating members acting on the former through one of said pair of apertured members, said resilient means acting between said fixed part and said first actuating member acting on the latter through the other of said pair of apertured members and, said pair of apertured members clamping said first conductive element therebetween.

11. An electromagnetic device or a power unit according to Claim 10 in which the resilient means acting between the fixed part and the first actuating member has a higher loading than the resilient means acting between the first and second actuating members.

12. An electromagnetic device or a power unit according to Claim 10 or 11 in which said second conductive element is mounted between two abutment means carried by said sleeve and comprising resilient means acting between said second element and the abutment means nearer the head portion of the first actuating member.

13. An electromagnetic device or a power unit as claimed in Claim 10, 11 or 12 in which each conductive element is located between two pairs of fixed contacts, each pair being arranged as aforesaid, and bridges one pair in the first position and the other pair in the second position of the contact assembly, and the displacement of the first conductive element from said first to said second position is less than the lost motion between said first and second actuating members.

1 4. An electromagnetic device or a power unit as claimed in any preceding claim in which there is lost motion between the movement of said core member and the actuation of said first actuating member.

15. A power unit as claimed in Claim 13, further comprising a terminal connectable to one terminal of a battery, one side of the ignition switch being connected to said terminal and the other side of the ignition switch being connected through one of said windings to a first contact of said one pair of fixed contacts associated with said second conductive element and to a first contact of said other pair of fixed contacts associated with said first conductive element, the second contact of said one pair of fixed contacts associated with said second conductive element being connected to ground, and the other side of the ignition switch also being connected to a first contact of the said one pair of fixed contacts associated with the first conductive element, the second contact of this last-mentioned pair being connected through to other winding to ground and being connected to the second contact of said other pair of fixed contacts associated with the first conductive element, whereby the two windings are connected in parallel in said first position and in series in the said position between said terminal and ground; and a first contact of said other pair of fixed contacts associated with said second conductive element being connected to said one side of the ignition switch, the second contact of this last-mentioned pair being connected to the starting motor to energize same in said second position.

16. A power unit as claimed in Claim 1 5 in which there is lost motion between the movement of said core member and the actuation of said first actuating member.

1 7. In or for the power unit whose prime mover is an internal combustion engine or other heat engine which has an electrically energisable starting motor for the engine, an electromagnetic device substantially as hereinbefore described with reference to the accompanying drawings.